1. Field of the Invention
The present invention relates, in general, to an image forming apparatus and an image forming method using an electrophotographic process and, in particular, to an image forming apparatus and an image forming method for forming a latent image by scanning the surface of a photosensitive substance with a blinking laser light based on an image signal.
2. Description of the Related Art
In general, in an image forming apparatus using a digital image signal, first of all, a photosensitive material serving as an image bearing body is electrically charged uniformly. Then, a laser light which is modulated by an image signal in picture-element units is applied to the surface of the image bearing body to form an electrostatic latent image thereon. Portions of the image bearing body on which the laser light has been applied have different potentials depending upon the amount of laser light applied thereto. On the latent image, toner is selectively transferred to form a toner image which is then transferred and fixed onto paper to form a recorded image.
In an image forming apparatus like the one described above, as a method to expose the image bearing body with the amount of exposure determined by image data of each picture element, a technique applying pulse-width modulation on a laser is widely used. In this technique, the amount of exposure is varied for each picture element by changing the exposure time duration with the output of the laser light kept at a constant value. An image drawing apparatus shown in FIG. 14 is an example of an image forming apparatus which adopts the pulse-width modulation method. The device enclosed by a dashed line in the figure is an exposure control unit 100 for controlling the exposure of the image drawing apparatus. In the image drawing apparatus, image data output by an image processing unit 101 is converted into an analog voltage signal by a D/A converter 103 in synchronization with a clock signal of FIG. 15A generated by a clock-signal generator 102. On the other hand, a chopping-wave oscillator 104 generates a chopping wave which is also synchronized with the clock signal. The signal output by the D/A converter 103 is compared with the signal output by the chopping-wave oscillator 104 by a comparator 105 in order to convert the image data into an exposure control signal with a pulse width varying from picture element to picture element as shown in FIG. 15B. A V/I converter 106 carries out control to turn on and off a driving current wave of a light emitting device in accordance with the exposure control signal. The driving current wave output by the V/I converter 106 is constant during the exposure time duration of each picture element as shown in FIG. 15C.
A semiconductor laser 108, the light emitting device cited above, is driven by the driving current wave which is controlled to turn on and off by the V/I converter 106 as described above, radiating a laser light. The laser light is converted into parallel beams by a collimator lens 109 and then polarized by a polygon mirror 110 prior to adjustment of the focus and the scan speed thereof by an f.theta. lens 111 before being applied to the surface of an image bearing body 112. Thereafter, a latent image formed on the image bearing body 112 is made visible by the generally known electrophotographic process and recorded on paper.
Since a laser beam traditionally used in the image drawing apparatus described above has a large beam diameter, a beam spot produced at the exposure time duration juts out into adjacent picture elements as shown in FIG. 16, giving rise to mutual interference among picture elements. The interference much affects an intermediate-gradation image, causing a problem that an image cannot be reproduced with a high degree of fidelity. In order to solve this problem, it is thus necessary to reduce the beam diameter. In addition, it is also necessary to raise the resolution in order to reproduce an image constructed by thin lines such as a character with a high degree of fidelity. Also, in this case, the beam diameter of the laser light needs to be reduced. In order to reduce the beam diameter by means of an optical system comprising lenses and mirrors, however, the lens configuration becomes complex and, in addition, the size of the polygon mirror increases, naturally making the image drawing apparatus large in size.
In order to solve the problems described above, there is provided an image forming apparatus wherein the laser beam is squeezed into a smaller beam diameter by using means such as a diaphragm placed between the polygon mirror and the collimator lens for shielding part of the optical path of the laser light as is disclosed in Japanese Unexamined Patent Publication No. Sho 61-113018 (1986).
When the beam diameter is reduced physically as described above, however, diffraction of light occurring in the shielding means gives rise to fringes around a main beam spot as shown in FIG. 17A. Photographic fogs caused by the fringes in turn cause the picture quality to deteriorate. A technology developed to prevent the picture quality from deteriorating due to photographic fogs is disclosed in Japanese Unexamined Patent Publication No. Hei 6-305189 (1994).
According to an image forming method disclosed in Japanese Unexamined Patent Publication No. Hei 6-305189, an image bearing body having an photoelectric characteristic shown in FIG. 17B is used. As shown in FIG. 17B, as the exposure intensity (that is, the exposure energy) exceeds a certain fixed value (a), the surface potential drops steeply. Accordingly, when a laser light is applied to the surface of the image bearing body, the surface potential does not attenuate for an exposure intensity smaller than the value (a) and only the surface potential of exposure portions with an exposure intensity greater than the value (a) does attenuate. That is to say, since the amount of attenuation in surface potential of the photosensitive material is small for a range with an exposure intensity smaller than the value (a), effects of fringes shown in FIG. 17A can be suppressed, allowing only a large peak to become a predominant factor in the distribution of the surface-potential attenuation of the image bearing body.
According to an image forming method disclosed in Japanese Unexamined Patent Publication No. Hei 4-336859 (1992), on the other hand, by superposing two laser beams on each other as shown in FIG. 18, a beam dot with an amount of exposure larger than that produced by a single beam can be formed. Then, by using an image bearing body with the photoelectric characteristic described above, dots each having a size smaller than the beam diameter can be formed at a high density. The image bearing body exhibits an photoelectric characteristic that, at an amount of exposure produced by superposing two beams, that is, at an amount of exposure greater than the value (a), the surface potential drops abruptly.
With the image forming method described above, however, the following problems arise.
With the image forming method disclosed in Japanese Unexamined Patent Publication No. Hei 6-305189, effects of fringes can be suppressed theoretically by using the photoelectric characteristic of the image bearing body. It is difficult, however, to make an image bearing body that has an extremely accurate photoelectric characteristic like the one shown in FIG. 17B and also has practically durable stability and maintainability as well.
On the of that, the image forming method disclosed in Japanese Unexamined Patent Publication No. Hei 4-336859 has a quality-deterioration problem such as photographic fogs and poorly reproduced highlight portions which problem arises when the development bias voltage or the like changes in addition to the problem described above. By the same token, there is a factor limiting the improvement of the resolution if the beam diameter is not reduced. In addition, since there is much mutual interference among picture elements, a complex image processing is required. As a result, there are a number of problems encountered in putting the image drawing apparatus to practical use.